This invention generally relates to the inspection of glass containers. More particularly, this invention relates to the inspection of glass containers for sidewall flaws using a television camera as a sensing element. Specifically, this invention relates to the inspection of glass containers for stress inducing sidewall inclusions by the use of polarized infra-red radiation.
It has long been known that stresses in glass containers could be seen by viewing the glass container through crossed polarizing filters. Solid inclusions in the sidewalls of glass containers are generally referred to as "stones". This general term is used regardless of the source of the inclusion. Stones in the sidewalls of glass containers create stress patterns which can be seen when the polarizing filters are used. The following paper suggested the use of a television camera and polarized illumination to detect stones in glass containers: "Automatic Stone Detector With Closed Circuit TV", M. Watonabe, Y. Ito, S. Nakatani, IFAC Symposium on Automatic Control in Glass, Lafayette, Indiana, 25-28, September, 1973, (Pittsburgh, Pa.: ISA 1973), p. 196-201. However, this system has not proven to be completely satisfactory because of ambient light interference problems, glass color problems, and glass surface texture interference. We have found that the use of polarized infra-red radiation will eliminate visible light problems, but still allow viewing of sidewall stresses caused by stones. Other examples of the prior art may be seen in the following U.S. Pat. Nos. 2,649,500; 2,798,605; 3,379,829; 3,565,536; 3,576,442; 3,746,784; and 3,894,806.